In an automatic leveling control, an irradiation direction can be changed by automatically adjusting the position of an optical axis of a vehicle headlamp according to an inclination angle of the vehicle. Generally, in automatic leveling control, vehicle height sensors are used as an inclination detection unit of a vehicle, and the position of the optical axis of a headlamp is adjusted on the basis of a pitch angle of the vehicle that is detected by the vehicle height sensors. Alternatively, an automatic leveling control may be implemented by using an acceleration sensor for detecting a pitch angle of the vehicle (see, e.g., JP2000-085459A, JP2004-314856A, JP2001-341578A and JP2009-126268A).
By using an acceleration sensor to detect a pitch angle of a vehicle, it is possible to reduce the manufacturing cost as well as the weight of an automatic leveling system as compared with a situation in which vehicle height sensors are used. Nevertheless, there is a demand for improved accuracy of an automatic leveling control using an acceleration sensor.